Chemical vapor deposition of monolayer-thin WS 2 crystals from the WF 6 and H 2 S precursors at low deposition temperature

Monolayer-thin WS with (0002) texture grows by chemical vapor deposition (CVD) from gas-phase precursors WF and H S at a deposition temperature of 450 °C on 300 mm Si wafers covered with an amorphous Al O starting surface. We investigate the growth and nucleation mechanism during the CVD process by analyzing the morphology of the WS crystals. The CVD process consists of two distinct growth regimes. During (i) the initial growth regime, a fast and self-limiting reaction of the CVD precursors with the Al O starting surface forms predominantly monolayer-thin WS crystals and AlF crystals that completely cover the starting surface. During (ii) the steady-state growth regime, a much slower, anisotropic reaction on the bottom, first WS layer proceeds with the next WS layer growing preferentially in the lateral dimensions. We propose that the precursor adsorption reaction rate strongly diminishes when the precursors have no more access to the Al O surface as soon as the WS layer completely covers the Al O surface and that the WS crystal basal planes and AlF crystals have a low reactivity for WF adsorption at 450 °C. Nonetheless, a second layer of WS starts to form before the first WS layer completely covers the starting surface, albeit the surface coverage of the second layer is low (